Boilers are used in a variety of applications and processes in the world today. One of the more common types of boilers, the water-tube boiler, uses heat from fuel burned within a combustion chamber to heat water circulating through a network of internal tubes. Water-tube boilers typically consist of two principal sections, a radiant section and a convective section. Some boilers are further equipped with a super heater mechanism for, inter alia, applications in which superheated steam is beneficial or required.
Package water-tube boilers are small to mid-sized water tube boilers that are preconstructed and assembled in a factory. These types of boilers can be shipped and installed as a complete unit, including an integrated burner, and do not require much more than fuel and water sources and appropriate ventilation.
A fundamental advantage of package boilers is an installed cost which is considerably lower than that of a field-erected boiler. This cost advantage is made possible by basic designs that allow standardized fabrication processes while still providing sufficient flexibility to permit satisfactory adaptation to the specific needs of a particular application. As a result, package boilers are typically constructed using standard, industry wide designs. Three of the most prevalent designs of package boilers are the “A”, “D”, and “O” types so named based upon the approximate shape of their respective tubes. In the conventional designs, the mud and steam drums are typically aligned. The drums may, however, be offset as disclosed in U.S. Pat. No. 6,901,887. The offset drum arrangement offers multiple advantages, including, maximizing heat transfer, better control and reduction of NOx emissions, and easier shipping of the pre-constructed unit. Through a modification of the tube arrangement and/or the addition of baffles, a multi-pass boiler can also be created.
The configuration of the tubes connecting the lower drum to the upper drum is especially important in a package boiler. These tubes must not only convey saturated steam and water to the upper drum, but must also adequately cool the unit and the walls in order for the boiler to have its small size. This is an important point as the space available within the unit for insulation is limited.
It would be advantageous to provide a package boiler with the highest operational efficiency while maintaining the smallest footprint. It is further desirable to accomplish such goals while reducing the overall manufacturing costs of the boiler unit.